--- title: "Short note: Telephone numbering in PSTN" subject: "Telecommunications" chapter: 7 key: "semester_8/telecommunications:short-note-pstn-numbering" source: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-pstn-numbering site: BCT Engineering --- # Short note: Telephone numbering in PSTN Telecommunications · Chapter 7: Telecommunication Regulation - Asked in: 2074 Asoj, 4 marks; 2071 Shrawan, 8 marks (asked 2 times) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-pstn-numbering - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-7 ## Question Write a short note on telephone numbering plan in PSTN (land line telephone exchange). ## Short answer The PSTN numbering plan follows ITU-T E.164, limiting international numbers to 15 digits. It comprises an international prefix (00), country code (e.g., 977 for Nepal), area code, and subscriber number. Nepal uses an open plan where the trunk prefix 0 is dialled only for long-distance calls; locally, just the subscriber number is used. ## Answer A **telephone numbering plan** in the PSTN is the scheme that gives every landline subscriber a unique number, so that exchanges can route and charge calls from the dialled digits. National plans follow ITU-T **E.164**, which limits an international number to 15 digits. #### Structure of a PSTN number ```text 00 977 1 44 12345 | | | | | IP CC area exch. line code code number <-- NSN = area+subscriber --> ``` - **International prefix (00):** to dial out of the country. - **Country code (CC):** e.g. 977 Nepal, 91 India. - **Trunk prefix (0):** dialled before the area code for a national long distance (STD) call. - **Area (trunk) code:** identifies the charging/numbering area, e.g. 01 Kathmandu, 061 Pokhara, 021 Biratnagar. - **Subscriber number:** exchange code + line number; the exchange code selects the local exchange, the line number selects the subscriber's line. #### Types of numbering 1. **Open plan:** a trunk prefix and area code are dialled only for calls outside the area; number lengths may differ (Nepal, UK). 2. **Closed plan:** the full number, of fixed length, is always dialled (USA 10 digits). #### Requirements - Unique and easy to remember. - Number length planned for future growth. - Digits allow routing and charging. - Special short codes for emergency and services (Nepal: 100 police, 101 fire, 102 ambulance, 197 directory). **Example (Nepal):** a Kathmandu subscriber 4412345 is dialled as 4412345 locally, 01-4412345 from another district, and +977-1-4412345 from abroad. In a large network the digits are analysed step by step: the first digit 0 sends the call to the trunk exchange, the area code selects the route, and the remaining digits select the local exchange and line. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the role of the trunk prefix 0 in a PSTN number? A. It identifies the country B. It tells the exchange a national long distance (STD) call follows C. It selects the subscriber line D. It marks emergency calls Answer: B. Dialling 0 sends the call to the trunk exchange for a call outside the local area. (Key point: Trunk prefix) 2. In the subscriber number, the exchange code and line number select respectively: A. the country and the area B. the area and the exchange C. the local exchange and the subscriber's line D. the subscriber and the country Answer: C. The exchange code picks the local exchange and the line number picks the line. (Key point: Parts of subscriber number) 3. In an open plan such as Nepal's: A. the full number is always dialled B. trunk prefix and area code are dialled only for calls outside the area C. only 10 digits are allowed D. no area codes exist Answer: B. Open plans use the trunk prefix and area code only for calls outside the local area, and lengths may differ. (Key point: Open plan) ## Related in Telecommunications - [NTA's role and ITU numbering plan](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Anta-role-itu-numbering) — Telecommunications, chapter 7 - [Working principle of a digital telephone exchange](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-telephone-exchange-principle) — Telecommunications, chapter 4, asked 4× - [Telephone exchange and switching hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-switching-hierarchy) — Telecommunications, chapter 1, asked 3× - [Telephone exchange structure; TST vs STS](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-structure-tst-sts) — Telecommunications, chapter 4 - [Charging plan in a telephone network](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Acharging-plan-telephone) — Telecommunications, chapter 7 - [IP and soft switching for PC-to-PSTN calls](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aip-soft-switching-skype-pstn) — Telecommunications, chapter 8 ## Related in other subjects - [Distortionless line and telephone lines](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Adistortionless-line-telephone) — Electromagnetism, chapter 5 - [Equivalence partitioning: telephone numbers](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Aequivalence-partitioning) — Software Engineering, chapter 8